The present invention generally relates to tail construction for heating elements. More specifically, the present invention relates to ceramic heating elements with tails constructed such that the tails heat to substantially lower temperatures than normal "hot tail" construction.
Ceramic heating elements are, of course, generally known and used in the construction industry. Typically, ceramic heating elements are used for pre-heating and post-weld stress relieving piping welds. The most common form of heating element consists of interlocking ceramic beads for electrical insulators forming a flat pad. Typically, the flat pads are approximately three-eighths (3/8") of an inch thick but may vary depending on the beads or other insulators used. Typically, the beads are strung together using 80/20 nichrome wire to form a continuous circuit. The two ends of the wire extend from the corners of a heating element consisting of strung-together ceramic beads. The ends of the wires extend for a sufficient distance to allow for electrical connection of the wires to a source.
The wire leads are commonly called "tails" in the industry and are designed as either "hot" tails or "cold" tails. Each type of tail, however, has its drawbacks.
Hot tail construction connects the resistant wire producing the heat inside the heating element directly to an electrical connector at the end of the tail. Typical electrical connectors are made from brass, but other connectors, of course, may be implemented. As a voltage is applied to the tails of the heating element, electrical current flows through the wire. Typical voltages of eighty volts are applied resulting in a current of approximately sixty Amperes thereby causing the temperature of the nichrome wire to rise. Since the nichrome wire is connected directly to the terminals, each of the terminals also become hot. This type of construction is beneficial in that a continuous wire is connected to the electrical connector resulting in no weak points in the heating element or the tails.
Alternatively, cold tails are provided having a different form of construction than hot tails. Cold tails are constructed so as to lower the resistance of the wire leads thereby lowering the temperature of the tail itself. Typically, cold tails are constructed by one of two methods.
One method of construction of a cold tail is to place a second nichrome wire adjacent to the existing wire in the tail. A metal tube is then slid over both wires up to the main heating element body and crimped to produce an electrical connection. Both of the wires are then run into the electrical connector, typically constructed from brass. Having two wires, the electrical resistance is only one-half of the original amount producing only one-fourth of the heat generated in a system using hot tail construction. Therefore, the connector is substantially cooler. However, this results in several drawbacks.
One drawback is that the tails are flexible and, therefore, the point at which the metal sleeve is crimped becomes a weak point. The crimped sleeve cuts the small individual strands of the heating wire causing a hot spot which fails earlier than any other point on the wire. A second drawback is that a manufacturer of heating elements must also stock and use a different ceramic bead to insulate the tail since the tail has a larger inside diameter than the outer beads needed for the remainder of the heating element.
Another type of cold tail construction involves butt welding of the nichrome wire at the point at which it exits the main heating element. The nichrome wire is butt welded to a nickel wire of the same size. Nickel wire has a substantially lower resistance than the nichrome wire resulting in a lower temperature at the connector. A butt weld, however, is typically difficult to construct. The two wires must be welded together without changing the diameter of the joint. Further, the weld must appear as one continuous wire. Any reduction in diameter causes a hot spot which fails early after use. Any increase in diameter prevents the ceramic insulators from sliding over the joint. Furthermore, any contamination in the weld causes the joint to heat up. Therefore, a clean environment is required to manufacture this type of cold tail.
A need, therefore, exists for an improved tail for heating elements and a method for heating overcoming the deficiencies of the known tail constructions and combining the advantages of both hot tail construction and cold tail construction.